Understanding the connectivity of the corpus callosum (CC) is an important public health goal, as this structure is believed to be fundamentally important to the emergence of higher order cognitive functions in humans. Furthermore, differences in the size of the CC have been associated with a number of neuropsychiatric and neurodevelopmental disorders such as schizophrenia, autism spectrum disorder, and Tourette's syndrome. Evaluating the proximate and ultimate mechanisms that account for variation in CC size is crucial in understanding its role in a variety of human cognitive and behavioral processes. In this application, we will examine the topography of the CC in two divergent primate species - chimpanzees (Pan troglodytes) and capuchin monkeys (Cebus apella) - that have independently evolved several behavioral and anatomical characteristics in common with humans. Specific Aims are to 1) map the anatomical connectivity of the CC in capuchin monkeys and chimpanzees in vivo using MRI and DTI;2) evaluate the histological organization of the CC in capuchin monkeys and chimpanzees using post-mortem specimens;3) validate DTI as a reliable and accurate marker of myelination and fiber orientation pathways of the CC;and 4) correlate behavioral measures of complex motor tasks with individual differences in CC size. We will combine brain imaging (MRI and DTI) and behavioral measures in socially living subjects (36 capuchin monkeys and 36 chimpanzees) alongside imaging and histological examination of post-mortem specimens (8 capuchin monkeys and 25 chimpanzees;behavioral data of these subjects'performance on motor tasks is available). We hypothesize chimpanzees (but not capuchin monkeys) will show similar topography to humans, with regions associated with the frontal cortex to account for approximately 2/3 of CC fibers. Capuchins are expected to have significantly less fiber connections to the frontal lobe. However, if the topography of the CC is conserved across the primate Order, then capuchins should show similar organization as chimpanzees (and humans). Additionally, we expect to find sex and handedness effects on CC topography, with females and left-handed individuals showing different composition of the fibers within the CC compared to males and right-handed individuals, respectively. We further hypothesize that subjects who show better inter-hemispheric transfer of motor learning will have a relatively larger CC. PUBLIC HEALTH RELEVANCE: This research will examine the topography of the corpus callosum to evaluate the proximate and ultimate mechanisms that account for variation in the size of this structure. Individual differences in the size of the corpus callosum have been associated with a number of neuropsychiatric and neurodevelopmental disorders including schizophrenia, autism, and Tourette's syndrome. Thus, evaluating the mechanisms that influence corpus callosum size is critical to further our understanding of psychiatric conditions and neurodevelopmental disorders.